Osteoporosis is a major public health problem mainly characterized by low bone mineral density (BMD). BMD has a heritability > 60%. Under the current funding support (R01AR050496) (8/1/04-3/31/08), we tested a number of candidate genes with known functional importance for association with BMD variation. These genes may account for some but less than 10% heritability of BMD. This status leaves genetic determination of the remaining ~50% BMD variation to be identified, a challenge we embark to undertake here. The GOALS of this competitive renewal application are to: 1) identify NOVEL osteoporosis genes; 2) assess their sex and ethnic specificity/generality; and 3) perform functional studies of the identified novel genes for their specific molecular functional mechanisms for BMD variation. This application builds upon the integration of the significant progress made by our multidisciplinary research team for this project (R01AR050496) and for our other ongoing projects (R01AG026564, R21AG027110, P50AR055081) for genetic/genomic/functional genomic/proteomic studies of osteoporosis. Two coherently integrated stages are proposed for an in-depth and comprehensive study. Stage 1 will identify novel genes with population and genomic/functional genomic statistical significance. Stage 2 will investigate the functional significance by studying specific molecular and cellular mechanisms of the identified novel genes. Stage 1 (Primary): Association studies of novel osteoporosis genes. Aim I: Identification of novel genes - from multiple sources by joint and comprehensive consideration of evidence from genomics/functional genomics/proteomics studies, including the data from others and from our own completed/ongoing whole genome linkage scan and fine mapping studies (R01AG026564), genome-wide association, transcriptome and proteome studies (R21AG027110, P50AR055081). Aim II: Recruit and phenotype 2,900 new unrelated Caucasian subjects, together with the 1,138 such subjects already recruited in our pilot recruitment study, we will have > 4,000 subjects for this study. Aim III: Candidate gene association analyses, for ~100 identified novel candidate genes for association with BMD variation in the 4,000 unrelated subjects and validate the significant association in 818 Caucasian nuclear families (already recruited by the current support R01AR050496). Aim IV: Replication studies, in Chinese Han, US Caucasians, Blacks, and Mexican Americans. Stage 2 (Secondary): Functional studies of molecular and cellular mechanisms of novel significant genes identified, to be exemplified by studying the HDC and RUNX1 genes. HDC and RUNX1 genes were recently identified in our functional genomics studies and showed significant association with BMD variation in our follow- up population association analyses. Aim I: To characterize the effects of individual potentially functional SNPs and haplotypes of HDC on gene expression and function, and their subsequent effects on osteoclastogenesis. Aim II: To characterize the role of RUNX1 gene in osteoclastogenesis and bone resorption. Identifying genes for human BMD variation and gaining insights into the fundamental molecular mechanisms of risk to osteoporosis are important for 1) discovering new pathways and targets for therapeutic cures; 2) identifying genetically susceptible individuals, so that future preventions and interventions can be targeted to and based on individuals' specific genotypes. PUBLIC HEALTH RELEVANCE: Despite extensive studies in the past decade on gene identification of osteoporosis, critical gaps remain: All the tested known functional candidate genes can explain no more than 10% BMD variation in any human population, though >60% of BMD variation is attributable to genetic factors. Clearly, novel genes await discovery, and their functional mechanisms needs to be illuminated. This proposed project intends to undertake the challenge to contribute to fill in the above gaps: 1) We will use an integrated and innovative genomic convergence approach that combines results from genetic epidemiology, functional genomics, proteomics, animal models and the latest in vivo and in vitro molecular genetic studies to identify novel BMD candidate genes and infer some of their functions (at mRNA and protein levels). This is NOVEL in the field. 2) We will use both powerful and robust association approaches to test identified NOVEL genes for their importance on BMD variation and risk to osteoporosis for potential sex- specific effects and ethnic-specific or general effects in major ethnic groups. 3) We will pursue functional studies on molecular and cellular mechanisms of significant NOVEL genes to be identified, which will be exemplified by studying two specific novel genes HDC and RUNX1 that we recently identified. We expect this proposed project to lead to the identification of major novel genes underlying BMD variation. This project will actively and organically interact with the data from GWLS, GWA, and functional genomics studies that we already obtained and those to be obtained in both Caucasians and Chinese populations. Identifying such genes and gaining insights into the fundamental molecular mechanisms of risk to osteoporosis are important for 1) discovering new pathways and targets for therapeutic cures; 2) identifying genetically susceptible individuals, so that early preventions and interventions can be targeted to and based on individuals' specific genotypes.